I've played more with spices for herbal tea blends than for C. sinensis teas, but lots of spices seem to do nicely in various infusions. I've added these whole, or lightly crushed, with good results: cinnamon, cardamom (pods or seeds), anise, saffron, cloves, allspice. I'm sure that there are others that would also be good used this way (for example, short pieces of vanilla bean, cut open), but I can't vouch for them. How much to use and which teas to spice is of course up to your taste buds.

I love to poke around asian grocery stores looking for novel ingredients. Once I found a small box of tea masala (Mangal brand I think) in our local Indian grocer. I tried it with black tea (milk and sugar) and although it was fine I had two issues with the flavour: Firstly I question the use of ground spices in tea. Powdered spices infuse rapidly and it would be very easy to use too much. They also stale quickly and loose their fresh aroma. Another problem is the gritty, powdery texture it creates. Obviously using your own selection of whole spices avoids this problem. Secondly, indian tea masalas commonly contain black pepper. Yes it does give masala chai a strong kick but I found this flavour to be a bit too aggressive for my liking. Adding whole peppercorns would tone down the heat somewhat but still provide that nice lemony aroma. I love to use Indian spices but mainly just to create my own curry masalas.